In a typical modem-to-modem communications system without error-correcting code (ECC), data packets are transmitted serially between the modems along with their embedded checksums. If the embedded checksum of a received data packet matches the checksum determined by the receiver modem ("received checksum"), the receiver modem deems the data packet to have been received correctly and the receiver modem will then transmit a signal to the transmitter modem which indicates that the data packet has been received correctly ("OK" signal). However, if the received checksum does not match the embedded checksum, the receiver modem deems the data packet to have been received incorrectly and the receiver modem will transmit a signal to the transmitter modem which indicates that the data packet has been received incorrectly ("not-OK" signal).
In a typical such modem-to-modem communications system, if the transmitter modem receives no response from the receiver modem or if the transmitter modem receives a "not-OK" signal, the transmitter modem will retransmit the data packet. Further, in such a typical system, such retransmission will continue until the transmitter modem receives an "OK" signal or until a predetermined time period has elapsed without the transmitter modem having received any signal.
As one can readily appreciate from the above, since incorrectly received data packets are discarded by the receiver modem and the data packet is retransmitted, the number of retransmissions may be high. One solution which is used to minimize such retransmissions which is well known in the art is to utilize an ECC in conjunction with the transmission of the data packets. However, this solution is incompatible with modem protocols which are used in existing modem-to-modem communications systems which do not utilize ECC. In addition, retrofitting such systems with an ECC capability may not be practical for at least two reasons: (a) there is typically a high cost associated with such retrofitting and (b) there is a lack of technical expertise in the industry relating to older communications systems which are comprised of modems that do not utilize ECC. Nevertheless, despite these problems in retrofitting, it is highly desirable, from an efficiency standpoint, to minimize retransmissions on modem-to-modem communications systems which do not utilize ECC.
As a result of the above, there is a need for method and apparatus for minimizing retransmissions between modems which do not utilize ECC and which, therefore, may be used in conjunction with older, transmitter modems which do not utilize ECC.